Scintillation cameras have, for several years, been used regularly as a clinical tool in diagnosing biological abnormalities. Tumors, lesions, and clotting can be readily located in many parts of the body of a patient being studied using a scintillation camera. U.S. Pat. No. 3,011,057 describes in detail the operation and function of a scintillation camera as envisioned by its inventor, Hal O. Anger. Use of a scintillation camera has rapidly expanded, as applications of the device were developed.
There have heretofore remained, however, certain limitation in connection with the use of a scintillation camera. A scintillation camera employs an array of photodetectors viewing overlapping portions of a scintillation crystal, usually formed of thallium-activated sodium iodide. The photodetectors produce electrical pulses in response to discrete quanta of radiation impinging upon the scintillation crystal. These pulses are processed by position computation circuitry, which registers the relative locations of quanta of radiation detected by the crystal for that radiation which lies within a specific energy range of interest. The position computation circuitry has heretofore been constructed so that processing of pulses emanating from different detected radioactive events have necessarily been performed sequentially. That is, pulses could be accepted and processed from only one quanta of radiation at a time. Since the processing of pulses requires a finite interval of time, any new pulses generated during such a finite interval are lost, since the computational circuitry is unable to accept them. THus, information from a portion of radioactive events occurring is irretrivably lost. In many instances, compensation for the resulting lost information has heretofore been effected by merely extending the duration of the clinical study. However, the information lost increases as a percentage of total radiation received with an increase in the rate at which quanta of radiation are detected by the scintillation crystal. That is, the percentage of information lost increases with level of radioactivity. At count rates above about 100,000 counts per second, conventional camera systems become oversaturated and fail to perform. The finite interval of time during which the position computation circuitry is unavailable for processing pulses from new radioactive events is called "dead time". This dead time is the same for all detected radioactive events in conventional commercial scintillation cameras, and is of the order of about 2.5 microseconds per detected event. This average dead time can be markedly reduced employing the combination of components of the present invention. This reduction in average dead time is achieved by accomplishing several objectives of the present invention.
One objective of the invention is to reduce the average dead time of a scintillation camera by prematurely terminating processing of those pulses which will eventually be rejected anyway. To this end, a preliminary differential discriminator is provided to reject at an early stage those pulses which clearly will eventually be found to be unacceptable for tabulation. Having thus reduced the dead time associated with clearly unacceptable pulses, intrinsic maximum rate of acceptance by the scintillation camera of pulses for processing is increased.
Another objective of the invention is to accommodate increases in radioactivity rate due to statistical variations in the number of quanta of radiation detected by the scintillation crystal. Accommodation of such increases without a loss in camera response sensitivity is achieved by providing a plurality of stages of buffer storage for electrical pulses produced in response to detected radiation. The position computation circuitry of the camera is thereby operative to accommodate statistical fluctuations in the rate at which pulses are received for processing, even when the nominal level of radiation received has reached the intrinsic maximum rate of acceptance of pulses for processing.
A further objective is to provide a means for detection and expulsion of pulses in the position computation circuitry which are generated in response to the detection of different quanta of radiation. Expulsion is carried out at an early stage in pulse processing to reduce the dead time in the circuitry attributable to the concurrent existence of such electrical pulses, the concurrent existence thereof being referred to as "pulse pile-up". Pulse pile-up is when a second pulse occurs before the first pulse has decayed. The second pulse occurs during the integrating time of the first pulse.
Another object of the invention is to prevent pulse distortion at high levels of radioactivity due to base line shift of capacitively coupled circuits by using DC coupling through all circuits. Pulse distortion is minimized by DC coupling of the photodetector amplifiers, by running the anodes of the photodetectors at ground potential and connecting the photocathodes thereof to a negative high voltage source. Direct coupling of the anodes to the amplifiers prevents the build-up of a base line voltage, especially when the system is processing a large number of pulses. Such a base line voltage results in distortion of pulses during shaping and integration, thereby contributing to decreased accuracy to pulses of interest. By employing the present invention, pulse amplitude distortion is alleviated and a greater number of pulses of interest and a fewer number of extraneous pulses are processed and registered.
Sensitivity to the energy range of interest is further improved by the provision of a means for automatic peak adjustment of the discriminator levels of the scintillation camera. A pulse energy peak of interest can thereby be tuned in better at the center of a pulse energy window. Adjustment of the window width is made in equal increments on either side of the center of the peak, rather than in one direction only from a single base line discriminator level.